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2847 Proceedings of the XVI ECSMGE Geotechnical Engineering for Infrastructure

2847 Proceedings of the XVI ECSMGE Geotechnical Engineering for Infrastructure and Development ISBN 978-0-7277-6067-8 © The authors and ICE Publishing: All rights reserved, 2015 doi:10.1680/ecsmge.60678 Impacts from groundwater control in urban areas Impacts de contrôle des eaux souterraines dans les zones urbaines M. Preene*1 and S. Fisher2 1 Preene Groundwater Consulting Limited, Wakefield, UK 2 Groundwater Engineering Pte Limited, Singapore, Singapore * Corresponding Author ABSTRACT Construction of deep structures, such as basements, road underpasses or metro rail systems often requires significant ground- water control measures. The presence of neighbouring structures and the typically sensitive hydrogeological setting beneath many cities means that impacts on groundwater conditions are often a concern. Potential impacts from groundwater control works can be categorised as: geotechnical impacts; contamination impacts; water dependent feature impacts; water resource impacts; and water discharge impacts. Where impacts are of potential concern a programme of baseline monitoring and operational monitoring should be implemented to allow the magnitude of impacts to be assessed. Possible mitigation measures to reduce or avoid impacts from groundwater control include: artifi- cial recharge; targeted groundwater cut-off walls; temporary cut-off walls; and, measures to protect individual receptors. RÉSUMÉ Construction de structures profondes, comme les sous-sols, les passages souterrains ou les systèmes ferroviaires de métro ont souvent besoin des mesures de contrôle des eaux souterraines importantes. La présence des structures avoisinantes et la situation hydrogéo- logique généralement sensible de nombreuses villes signifie que impactes sur les eaux souterraines sont souvent un sujet de préoccupation. Les impacts potentiels de travaux de contrôle des eaux souterraines peuvent être classés de manière suivante: impacts géotechniques; im- pacts de la contamination; impacts de caractéristiques dépendantes de l'eau; impacts sur les ressources d’eau; et les impacts d'évacuation d'eau. Lorsque les impacts sont une préoccupation potentielle un programme de surveillance de base et opérationnel doit être mis en œuvre pour permettre l’évaluation de l'ampleur des impacts. Les mesures d'atténuation possibles pour réduire ou éviter les impacts de contrôle des eaux souterraines comprennent: la recharge artificielle; murs parafouille hydrogéologiques ciblées; murs parafouille temporaires; et des me- sures pour protéger les récepteurs individuels. 1 INTRODUCTION Groundwater control is an important part of many civil engineering projects where excavation is made below groundwater level, for example for deep basements or for transport infrastructure (e.g. road underpasses or metro rail systems). An extensive range of groundwater control techniques is available to allow excavations to be made in a wide range of ground conditions and hydrogeological settings (Cashman & Preene 2012). Traditionally, the primary focus of geotechnical engineers and construction managers designing and implementing groundwater control systems has been on providing safe and robust solutions within the constraints of available budget, resources and sched- ule. In the past, it was not routine to consider the po- tential environmental impacts of dewatering and groundwater control. In recent decades, discussion and guidance has emerged on potential environmen- tal impacts (such as Powers 1985; Preene & Brass- ington 2003), and these issues are now routinely con- sidered in most major civil engineering projects. Indeed, in areas such as the Middle East, where there are major infrastructure construction programmes re- quiring groundwater control, regulatory guidelines stress the importance of avoiding or mininising im- pacts (Abu Dhabi City Municipality 2014; ASHGH- AL 2014). Geotechnical Engineering for Infrastructure and Development 2848 This paper describes the particular issues and po- tential environmental impacts associated with groundwater control in urban areas, where the re- stricted working space and the presence of neigh- bouring structures will influence the choice of meth- ods, and discusses the requirements for environmental monitoring and mitigation. 2 METHODS OF GROUNDWATER CONTROL Groundwater control can be achieved via two princi- pal types of methods: pumping methods and exclu- sion methods. A range of methods can be used to control groundwater as shown in Table 1. Table 1. Groundwater control methods Pumping methods Exclusion methods Sump pumping Vertical wellpoints Horizontal wellpoints Deep wells with sub- mersible pumps Ejector wells Passive relief wells Electro-osmosis Steel sheet-piling Vibrated beam walls Cement-bentonite or soil- bentonite slurry walls Concrete diaphragm walls Bored pile walls Grout curtains (permeation grout- ing; rock grouting; jet grouting; mix-in place methods) Artificial ground freezing The techniques most commonly associated with groundwater control are the pumping methods. These involve pumping groundwater from an array of wells or sumps (Figure 1) with the aim of temporarily low- ering groundwater levels to allow excavations to be made in stable conditions. Pumping methods are also known as groundwater lowering, construction de- watering or simply dewatering. The amount of lower- ing of the groundwater level is known as drawdown. Figure 1: Groundwater control by pumping In contrast, exclusion methods rely on low perme- ability cut-off walls around the excavation to exclude groundwater from the excavation (Figure 2). Exclu- sion methods can significantly reduce, or even elimi- nate completely, the requirement to pump groundwa- ter. Each group of methods (pumping and exclusion) has the potential to cause different types of environ- mental impacts. Figure 2: Groundwater control by exclusion 3 INDICATIVE FACTORS FOR IMPACTS It would be extremely useful to practicing engineers if generic ‘key indicators’ of potential impacts from groundwater control could be identified. This could allow early screening of projects to determine wheth- er groundwater control works have the potential to cause significant impact, and therefore whether spe- cial monitoring and mitigation measures may be re- quired. Unfortunately, it is difficult to provide general in- dicators of the risk of potential impacts. This is partly because the potential impacts are largely controlled by the site setting and are dependent on hydrogeolog- ical conditions, which are unique to each site. Addi- tionally, the type of groundwater control methods used can also have key influence on the potential im- pacts. It is true that the scale and duration of the groundwater control works have some influence – as a general guide, excavations that go deeper below groundwater level and require groundwater control for long periods have the greater potential to create impacts than shallower excavations that are open for shorter time periods. This is particularly the case for 2849 groundwater control using pumping, where deeper, long-term excavations will tend to create a larger ‘zone of influence’ within which groundwater levels are lowered by the effect of pumping. However, it is important to realise that in the past there have been several cases where significant det- rimental impacts have occurred from groundwater control for shallow and short-term excavations. Where groundwater control is carried out in sparsely developed areas, even where detrimental impacts are expected, the potential consequences may be modest, because there are relatively few buildings or limited infrastructure around that may be affected. One of the particular challenges of working in densely developed urban areas is that the project site is likely to be surrounded by multiple neighbour- ing sites, which may have different sensitivities and vulnerabilities to impacts from groundwater control. 4 CATEGORIES OF IMPACTS The potential for impacts will have to be assessed on a site-by-site basis. Ideally such assessments need some structure of framework, and the categories of impacts in Table 2 are suggested as an aid to plan- ning and structuring the assessments. Table 2. Categories of impacts from groundwater control Impact category Types of impact Geotechnical Ground settlement – effective stress Ground settlement – loss of ground Contamination Mobilisation by pumping Creation of vertical flow pathways Water feature Reduction in flow Change in water quality Change in water level Water resource Change in water availability Change in water quality Water discharge Change in water quality Downstream scour and flooding The impacts categories in Table 1 are the direct im- pacts from interference with, or manipulation of, the groundwater regime. There will be additional indirect impacts (such as noise, emissions from plant, etc.) associated with the physical construction activities, such as well drilling or pumping. These indirect im- pacts are not discussed here. 4.1 Geotechnical impacts Defined as impacts where the geotechnical properties or state of the ground are changed by groundwater control activities. The most common type of impact in this category is ground settlement, with the corresponding risk of distortion and damage to structures, services and oth- er sensitive infrastructure. Ground settlement can be caused by two principal mechanisms: ¥ Increases in effective stress as a result of lower- ing of groundwater levels, resulting in com- pression and consolidation of the ground. Such settlements are an unavoidable consequence of lowering of groundwater levels. ¥ Removal of fine particles from the ground (loss of fines) which can occur when poorly con- trolled sump pumping draws out soil particles with the pumped water. With good design and implementation, loss of fines (and the associat- ed settlement risk) can be avoided. Further details on settlement caused by groundwa- ter control works are given in Preene (2000). Lowering of groundwater levels can potentially cause other geotechnical impacts. One possible im- pact is negative skin friction loads on piles (where soil is consolidating around the piles). Another pos- sible impact is the degradation of timber piles sup- porting older structures. This uploads/Ingenierie_Lourd/ preene-and-fisher-2015-impacts-from-groundwater-control-in-urban-areas.pdf